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The first trimester of pregnancy is calculated from 1 week to 12 weeks of gestation. It is in the first trimester that fetal development begins in the womb.
The following is a review of your little one’s development in the womb, from the first week to the 12th week!
Fetal development during the first trimester of pregnancy
Fetal development usually follows a predictable course. According to the Mayo Clinic , the following is fetal development that occurs during the first trimester.
Weeks 1 and 2
Fertilization usually occurs about two weeks after your last menstruation begins. To calculate your estimated due date, your doctor will count the next 40 weeks from the start of your last period.
This means your period counts as part of your pregnancy, even if you are not pregnant at the time.
3rd week
Sperm and egg cells unite in one of the fallopian tubes to form a single-celled entity called a zygote. If more than one egg is released and fertilized or if the fertilized egg splits in two, you may have multiple zygotes.
A zygote usually has 46 chromosomes, 23 from the biological mother and 23 from the biological father. These chromosomes help determine the gender and physical characteristics of your future baby.
Immediately after fertilization, the zygote moves down the fallopian tube towards the uterus. At the same time, it will begin to divide to form a cluster of cells that resembles a small raspberry or morula.
4th week
The ball of rapidly dividing cells now known as the blastocyst has begun burrowing into the uterine lining ( endometrium ). This process is called implantation.
Inside the blastocyst, the inner group of cells will become the embryo. The outer layer will form part of the placenta, which will nourish the baby during pregnancy.
5th week
In the fifth week of pregnancy, or the third week after fertilization, levels of the hCG hormone produced by the blastocyst increase rapidly. This signals the ovaries to stop releasing eggs and produce more estrogen and progesterone.
Elevated levels of this hormone stop menstrual periods, which are often the first sign of pregnancy, and trigger placental growth. The embryo is now made of three layers namely:
- The top layer or ectoderm will form the baby’s outer skin layer, central and peripheral nervous system, eyes and inner ear.
- The middle layer or mesoderm will form the baby’s heart and blood circulatory system. This layer of cells also serves as the foundation of the baby’s bones, ligaments, kidneys, and much of the baby’s reproductive system.
- The inner layer or endotherm is where the baby’s lungs and intestines develop.
6th week
In the 6th week of pregnancy in the first trimester, your little one will start to experience quite rapid growth. Four weeks after fertilization, the neural tube along the baby’s back will close.
The baby’s brain and spinal cord will develop from the neural tube. The heart and other organs also begin to form and the heart begins to beat.
The structures necessary for the formation of eyes and ears develop. Small shoots appear that will soon become arms. The baby’s body begins to experience a C-shaped curvature.
7th week
Seven weeks into pregnancy, or five weeks after conception, the baby’s brain and face begin to develop, Moms. At this stage the nostrils become visible, and the retina begins to form.
The lower leg buds that will become legs appear and the arm buds that grew last week are now paddle-shaped.
8th week
In the 8th week of pregnancy in the first trimester or six weeks after conception, the baby’s lower leg buds are paddle-shaped. Fingers begin to form.
Small swellings outlining the shell-shaped part of the baby’s future ear develop and his eyes become clear. The upper lip and nose have formed. The trunk and neck begin to straighten.
By the end of this week, the baby may be about 1/2 inch (11 to 14 millimeters) long from head to rump.
9th week
In the ninth week of pregnancy, or seven weeks after conception, the baby’s arms grow and elbows appear. Toes are visible and eyelids form.
By the end of this week, the baby may be less than 3/4 inch (16 to 18 millimeters) long from crown to rump.
10th week
In the 10th week of pregnancy in the first trimester, or 8 weeks after conception, the baby’s head will become rounder.
The baby can now bend his elbows. Toes and fingers become longer. The eyelids and outer ears continue to develop. The umbilical cord is clearly visible.
11th week
At the beginning of the 11th week of pregnancy in the first trimester, or the ninth week after fertilization, the baby’s head is still about half its length.
The baby is now officially referred to as a fetus. This week the baby’s face looks wide, the eyes are wide apart, the eyelids are together and the ears are set low. Buds for teeth begin to appear.
Red blood cells begin to form in the baby’s liver. At the end of this week, the baby’s external genitals will begin to develop into a penis or clitoris and labia majora.
Now the baby is probably about 2 inches (50 millimeters) long from crown to rump, and weighs almost 1/3 ounce (8 grams).
12th week
In the final stages of the first trimester of pregnancy, or 10 weeks after conception, the baby’s nails begin to grow. Your baby’s face has now taken on a more developed profile.
Now the baby is probably about 2 1/2 inches (61 millimeters) long from head to rump, and weighs about 1/2 ounce (14 grams).
It is during the first trimester that the fetus is most susceptible to damage from substances such as alcohol, drugs, certain medications, and diseases, such as rubella (German measles).
Moms’ body changes during the first trimester of pregnancy
During the first trimester of pregnancy, the bodies of the mother and fetus change rapidly. Here are some changes that Moms might feel during the first trimester of pregnancy!
- Enlarged mammary glands cause the breasts to swell and become tender in preparation for breastfeeding. This is due to an increase in the amount of the hormones estrogen and progesterone. Choose a bra with the right size, Moms.
- The areola (the pigmented area around each nipple) will enlarge and become dark. They may be covered with small white bumps called Montgomery tubercles (the result of enlarged sweat glands).
- Blood vessels become more visible on the surface of the breast.
- The uterus grows and begins to press on the bladder. This causes Moms to urinate more often.
- Due to the surge in hormones, you may experience mood swings similar to premenstrual syndrome (PMS) . This is a condition experienced by some women that is characterized by mood swings, irritability, and other physical symptoms that occur shortly before each menstrual period.
- Increased hormone levels to support pregnancy can cause morning sickness. However, morning sickness does not always occur in the morning.
- Constipation can occur when the growing uterus presses on the rectum and intestines.
- Muscle contractions in the intestines, which help move food through the digestive tract, slow down due to high levels of progesterone. This can cause heartburn, indigestion, constipation, and gas.
- Clothing may feel tighter around the breasts and waist, as the stomach begins to increase in size to accommodate the growing fetus.
- Moms may experience extreme fatigue due to the physical and emotional demands of pregnancy.
- Heart volume increases by around 40-50 percent from the beginning to the end of pregnancy. This causes an increase in cardiac output. Increased cardiac output can cause an increase in heart rate during pregnancy. Increased blood volume is needed for extra blood flow to the uterus.
During your first trimester of pregnancy, you have to be extra careful to maintain your body’s health, because your condition can also have an impact on your baby. Don’t forget to regularly consult a doctor, okay?
Consult Moms’ health problems during the first trimester of pregnancy through Good Doctor, a 24/7 service. Our doctor partners are ready to provide solutions. Come on, download the Good Doctor application here !
